weirdly, the first time i went exploring, i found a load of them & numerous water worlds, terraforming candidates, and GG's with life. made an absolute killing - well over a million and i'd only gone to the Plaedies and back, and only bothered scanning when i saw possible moneymakers / GG's...so i sstarted thinking they were pretty common.
the next time i went exploring, spent probably twice as long and far and found only one waterworld and maybe a half dozen GG's with life. that's it. probably less than 200k all in made.
so i guess i just got really lucky first time round...!